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Superconductors are well known for their ability to screen out magnetic fields. In type-II superconductors, as the magnetic field pressure is progressively increased, magnetic flux accumulates at the periphery of the sample, very much like charges accumulate in a capacitor when voltage is increased. As for capacitors, exceeding certain threshold field causes the blocked magnetic flux to abruptly penetrate into the sample. This phenomenon, triggered by a thermomagnetic instability, is somewhat analogous to the dielectric breakdown of the capacitor and leaves behind a similar Lichtenberg imprinting. Even though electrical breakdown threshold has been extensively studied in dielectrics, little information is known about the statistical distribution of the thermomagnetic breakdown in superconductors. In this work, we address this problem by performing magneto-optical imaging experiments on a Nb film where nanometric heating elements are used to rapidly erase the magnetic history of the sample. We demonstrate that the size and shape distributions of avalanches permits to unambiguously identify the transition between two regimes where either thermal diffusivity or magnetic diffusivity dominates. Clear criteria for discriminating athermal dynamic avalanches from thermally driven avalanches are introduced. This allows us to provide the first precise determination of the threshold field of the thermomagnetic breakdown and unveil the details of the transition from finger-like magnetic burst to dendritic branching morphology. These findings open a new avenue in the interdisciplinary exploration of catastrophic avalanches through non destructive repeatable experiments.

In this work, we show that targeted and controlled modifications of the Josephson-junction properties of a bridge-type $\mathrm{Nb}$ nanoSQUID can be achieved by an electroannealing process allowing us to tune and tailor the response of a single device. The electroannealing consists in substantial Joule heating produced by large current densities followed by a rapid temperature quench. We report on a highly nontrivial evolution of the material properties when performing subsequent electroannealing steps. As the current density is increased, an initial stage characterized by a modest improvement of the superconducting critical temperature and normal-state conductivity of the bridges, is observed. This is followed by a rapid deterioration of the junction properties, i.e., decrease of critical temperature and conductivity. Strikingly, further electroannealing leads to a noteworthy recovery before irreversible damage is produced. Within the electroannealing regime where this remarkable resurrection of the superconducting properties are observed, the nanoSQUID can be operated in nonhysteretic mode in the whole temperature range and without compromising the critical temperature of the device. The proposed postprocessing is particularly appealing in view of its simplicity and robustness.

International audience; Semiconductor–superconductor hybrid systems have outstanding potential for emerging high-performance nanoelectronics and quantum devices. However, critical to their successful application is the fabrication of high-quality and reproducible semiconductor–superconductor interfaces. Here, we realize and measure axial Al–Ge–Al nanowire heterostructures with atomically precise interfaces, enwrapped by an ultrathin epitaxial Si layer further denoted as Al–Ge/Si–Al nanowire heterostructures. The heterostructures were synthesized by a thermally induced exchange reaction of single-crystalline Ge/Si core/shell nanowires and lithographically defined Al contact pads. Applying this heterostructure formation scheme enables self-aligned quasi one-dimensional crystalline Al leads contacting ultrascaled Ge/Si segments with contact transparencies greater than 96%. Integration into back-gated field-effect devices and continuous scaling beyond lithographic limitations allows us to exploit the full potential of the highly transparent contacts to the 1D hole gas at the Ge–Si interface. This leads to the observation of ballistic transport as well as quantum confinement effects up to temperatures of 150 K. Low-temperature measurements reveal proximity-induced superconductivity in the Ge/Si core/shell nanowires. The realization of a Josephson field-effect transistor allows us to study the subgap structure caused by multiple Andreev reflections. Most importantly, the absence of a quantum dot regime indicates a hard superconducting gap originating from the highly transparent contacts to the 1D hole gas, which is potentially interesting for the study of Majorana zero modes. Moreover, underlining the importance of the proposed thermally induced Al–Ge/Si–Al heterostructure formation technique, our system could contribute to the development of key components of quantum computing such as gatemon or transmon qubits

Superconductors are well known for their ability to screen out magnetic fields. In type-II superconductors, as the magnetic field pressure is progressively increased, magnetic flux accumulates at the periphery of the sample, very much like charges accumulate in a capacitor when voltage is increased. As for capacitors, exceeding certain threshold field causes the blocked magnetic flux to abruptly penetrate into the sample. This phenomenon, triggered by a thermomagnetic instability, is somewhat analogous to the dielectric breakdown of the capacitor and leaves behind a similar Lichtenberg imprinting. Even though electrical breakdown threshold has been extensively studied in dielectrics, little information is known about the statistical distribution of the thermomagnetic breakdown in superconductors. In this work, we address this problem by performing magneto-optical imaging experiments on a Nb film where nanometric heating elements are used to rapidly erase the magnetic history of the sample. We demonstrate that the size and shape distributions of avalanches permits to unambiguously identify the transition between two regimes where either thermal diffusivity or magnetic diffusivity dominates. Clear criteria for discriminating athermal dynamic avalanches from thermally driven avalanches are introduced. This allows us to provide the first precise determination of the threshold field of the thermomagnetic breakdown and unveil the details of the transition from finger-like magnetic burst to dendritic branching morphology. These findings open a new avenue in the interdisciplinary exploration of catastrophic avalanches through non destructive repeatable experiments.

Local polarization of magnetic materials has become a well-known and widely used method for storing binary information. Numerous applications in our daily life such as credit cards, computer hard drives, and the popular magnetic drawing board toy, rely on this principle. In this work, we review the recent advances on the magnetic recording of inhomogeneous magnetic landscapes produced by superconducting films. We summarize the current compelling experimental evidence showing that magnetic recording can be applied for imprinting in a soft magnetic layer the flux trajectory taking place in a superconducting layer at cryogenic temperatures. This approach enables the ex situ observation at room temperature of the imprinted magnetic flux landscape obtained below the critical temperature of the superconducting state. The undeniable appeal of the proposed technique lies in its simplicity and the potential to improve the spatial resolution, possibly down to the scale of a few vortices.

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International audience; This paper presents a new class of high-performance permanent micro-magnets based on the controlled assembly of cobalt nanorods for the electromagnetic transduction of MEMS. Micromagnets are fabricated using a low temperature fabrication process that yields a dense material exhibiting high coercive field and remanence to saturation magnetization ratio. The cartography of the magnetic induction produced by the sub-millimeter size magnets was obtained using a scanning Hall effect micro-probe microscope. Silicon microcantilevers placed in the vicinity of these magnets were successfully actuated using the Lorentz force with low currents. The good signal to noise ratio measured at resonance demonstrates the potentiality of these nanostructured micro-magnets.

As the most sensitive magnetic field sensor, the superconducting quantum interference device (SQUID) became an essential component in many applications due to its unmatched performance. Through recently achieved miniaturization, using state-of-the-art fabrication methods, this fascinating device extended its functionality and became an important tool in nanomaterial characterization. Here, we present an accessible and yet powerful technique of targeted atom displacement in order to reduce the size of the weak links of a DC nano-SQUID beyond the limits of conventional lithography. The controllability of our protocol allows us to characterize in situ the full superconducting response after each electromigration step. From this in-depth analysis, we reveal an asymmetric evolution of the weak links at cryogenic temperatures. A comparison with time resolved scanning electron microscopy images of the atom migration process at room temperature confirms the peculiar asymmetric evolution of the parallel constrictions. Moreover, we observe that when electromigration has sufficiently reduced the junction's cross section, superconducting phase coherence is attained in the dissipative state, where magnetic flux readout from voltage becomes possible.

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer., Comment: 21 pages, including 2 pages of supplementary material

We demonstrate the in situ engineering of superconducting nanocircuitry by targeted modulation of material properties through high applied current densities. We show that the sequential repetition of such customized electro-annealing in a niobium (Nb) nanoconstriction can broadly tune the superconducting critical temperature T-c and the normal-state resistance R-n in the targeted area. Once a sizable R-n is reached, clear magneto-resistance oscillations are detected along with a Fraunhofer-like field dependence of the critical current, indicating the formation of a weak link but with further adjustable characteristics. Advanced Ginzburg-Landau simulations fully corroborate this picture, employing the detailed parametrization from the electrical characterization and high resolution electron microscope images of the region within the constriction where the material has undergone amorphization by electro-annealing.

International audience; The electromigration process has the potential capability to move atoms one by onewhen properly controlled. It is therefore an appealing tool to tune the cross sectionof monoatomic compounds with ultimate resolution or, in the case of polyatomiccompounds, to change the stoichiometry with the same atomic precision. Asdemonstrated here, a combination of electromigration and anti-electromigration canbe used to reversibly displace atoms with a high degree of control. This enables afine adjustment of the superconducting properties of Al weak links, whereas in Nbthe diffusion of atoms leads to a more irreversible process. In a superconductor witha complex unit cell (La2−xCexCuO4), the electromigration process acts selectively onthe oxygen atoms with no apparent modification of the structure. This allows to adjustthe doping of this compound and switch from a superconducting to an insulatingstate in a nearly reversible fashion. In addition, the conditions needed to replacefeedback controlled electromigration by a simpler technique of electropulsing arediscussed. These findings have a direct practical application as a method to explorethe dependence of the characteristic parameters on the exact oxygen content and pavethe way for a reversible control of local properties of nanowires.

The electromigration process has the potential capability to move atoms one by one when properly controlled. It is therefore an appealing tool to tune the cross section of monoatomic compounds with ultimate resolution or, in the case of polyatomic compounds, to change the stoichiometry with the same atomic precision. As demonstrated here, a combination of electromigration and anti-electromigration can be used to reversibly displace atoms with a high degree of control. This enables a fine adjustment of the superconducting properties of Al weak links, whereas in Nb the diffusion of atoms leads to a more irreversible process. In a superconductor with a complex unit cell (La

International audience; The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps to protect the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/μ 0 σ t, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We investigate this effect by studying Nb samples with a thickness step. Some of the observed features, namely the deflection of the flux trajectories at the border of the thick center, as well as the favored flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations.

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories.

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8 H 1 , H 1 and 1.6 H 1 , where H 1 corresponds to one flux quantum per pinning site. The vortex pattern at 0.8 H 1 corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6 H 1 the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6 H 1 interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ($d$ lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent $n$. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities.

We present a Scanning Hall Probe Microscope operating in ambient conditions. One of the unique features of this microscope is the use of the same stepper motors for both sample positioning as well as scanning, which makes it possible to have a large scan range (few mm) in x and y directions, with a scan resolution of 0.1 $\mu$m. Protocols have been implemented to enable scanning at different heights from the sample surface. The z range is 35 mm. Microstructured Hall probes of size 1-5 $\mu$m have been developed. A minimum probe-sample distance \textless{} 2 $\mu$m has been obtained by the combination of new Hall probes and probe-sample distance regulation using a tuning fork based force detection technique. The system is also capable of recording local B(z) profiles. We discuss the application of the microscope for the study of micro-magnet arrays being developed for applications in micro-systems. * gorky.shaw@ulg.this http URL.; Present address

Condon domains form when the de Haas-van Alphen (dHvA) amplitude becomes large compared to the dHvA period, i.e. μ0∂M/∂B > 1, where M is the magnetization and B the induction. This equation defines the boundary between the homogeneous magnetization and the Condon domain state. The phase diagram in the (B, T) plane can be predicted using the Lifshitz-Kosevich formula. We present the Condon domain phase diagram for a silver single crystal measured in magnetic fields up to 28 T and temperatures Tdown to 1.3 K. A standard ac method with a pickup coil system is used at low frequency for the measurements of the de Haasvan Alphen (dHvA) e..ect. The transition point from the state of homogeneous magnetization to the inhomogeneous Condon domain state is found as the point where a small irreversibility in the dHvA magnetization arises, as manifested by an extremely nonlinear response in the pickup voltage showing threshold character. The experimental results are in good agreement with theoretical predictions based on the Lifshitz-Kosevich formula.

The nonlinear response to an ac magnetic field with and without parametric pumping was studied experimentally on a rod-like beryllium single crystal at low temperatures in a quantizing magnetic field applied parallel to the hexagonal axis. At low temperatures (⩽3K) giant parametric amplification of the nonlinear response is observed in narrow windows near de Haas-van Alphen (dHvA) beating antinodes, where the amplitude of the magnetic oscillations is maximal. This effect is of threshold character with respect to the amplitude of the dHvA oscillations and occurs as a result of a diamagnetic phase transition of a strongly correlated electron gas into nonuniform state, with the formation of Condon domain structure. It is shown that the nonuniform phase appears periodically in a magnetic field. The components of the rectified signal field reproduce the envelope of H-T critical curves and can be used for reconstruction of diamagnetic phase diagrams.

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (μMI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test μMI structure made with spherical NdFeB particles of average particle size 16 μm is in good agreement with simulated field values (150 mT at 5 μm). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such μMI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The μMI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Vortex patterns at very low vortex densities in high quality MgB 2 single crystals are studied by scanning SQUID microscopy. In observations, characteristically vortices tend to line up locally at very low vortex densities and then form vortex cluster structure with increasing applied field. The evolution of vortex clusters is elucidated by molecular dynamics (MD) simulations based on a two-gap Ginzburg–Landau (GL) theory with type-1.5 conditions. In both observations and simulations, the average intervortex distance in clusters is independent of the applied field. The agreement confirms that high quality MgB 2 single crystals lie in the type-1.5 regime.

Vortices confined to micro-sized thin superconducting Pb squares are visualized using scanning Hall probe microscopy (SHPM). High resolution images are obtained after field cooling to 4.2 K for different magnetic fields using a standard AC-method for the Hall voltage readout. Vortex configurations for vorticity 1, 2 and 3 are observed and compared with theoretical predictions for the vortex patterns.

We present the experimental observation of hysteresis in high quality single crystals with different sample shapes of the simple metal beryllium. The hysteresis itself turns out to be very small. Here, Hall probe measurements at very low modulation frequency prove that the hysteresis is not due to eddy current effects. Furthermore, the hysteresis effect is measured using a standard AC method with various modulation levels and frequencies, and for samples with different demagnetization factors. We demonstrate that previously in rod-like samples observed effect is also present in a plate-like sample. The irreversible behavior of the magnetization occurs at the paramagnetic part of the de Haas–van Alphen (dHvA) period in case of Condon domain formation arising if the dHvA amplitude is strong enough. Moreover, we show that the resulting, extremely nonlinear response to a very small modulation field offers a simple way to find the Condon domain phase diagram.

In type-II superconductors, the flux-free state (Meissner state) may be invaded by vortices bearing quantized flux once H is above the lower critical field Hc1. However, the actual first flux penetration does not occur at Hc1 due to the presence of a surface barrier and the fact that the Meissner state may also exist as a metastable state up to a larger (superheating) field. In this work we determine the field for the first vortex penetration in superconductors by directly imaging the first vortex threading a superconducting Pb film with antidots. We find that the first vortex penetration occurs when the surface superconducting currents reach the depairing current, locally breaking superconductivity and allowing a vortex to nucleate. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:88 issue:18 status: published

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combing the phase sensitive character of ac-susceptibility, allowing to distinguish between the inductive-and dissipative response, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at timescales comparable to the period of the driving force (i.e. 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states., 5 pages, 2 figures

International audience; If a magnet of microscopic dimensions is brought in close proximity to a superconductor, the quantized nature of their interaction due to the creation of flux quanta in the superconducting system becomes noticeable. Herein, we directly image, via scanning Hall microscopy, the vortex-antivortex pairs in a superconducting film created by micromagnets. The number of antivortices at equilibrium conditions can be changed either by tuning the magnetic moment of the magnets or by annihilation with externally induced vortices. We demonstrate that small ac field excitations shake the antivortices sitting next to the micromagnets whereas no sizable motion is observed for the vortices sitting on top of the magnets, clearly revealing the different mobility of these two vortex species. A metastable state, which is obtained by applying a field after the system has been cooled down below the superconducting transition, shows a complex graded distribution of coexisting vortices and antivortices forming an intertwined critical state.

We studied experimentally the effect of a stripe-like domain structure in a ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the superconducting material, the regions of the reverse-domain superconductivity in the H-T plane can be isolated or can overlap (H is the external magnetic field, T is temperature). The latter case corresponds to the condition B_0/H_{c2}, 11 pages, 12 figures

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first penetration field, we directly observe a terraced critical state as proposed theoretically by Cooley and Grishin [Phys. Rev. Lett. 74, 2788 (1995)]. However, at higher fields, the flux front tends to disorder and the classical Bean profile is restored. This study allows us to establish a clear link between the widely used integrated response measurements in the superconducting state and the nanoscale landscape defined by individual vortices. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:83 issue:2 pages:1-4 status: published

International audience; We present the Condon domain phase diagram for a silver single crystal measured in magnetic fields up to 28 T and temperatures down to 1.3 K. A standard ac method with a pickup coil system is used at low frequency for measuring the de Haas-van Alphen effect (dHvA). The transition point from the state of homogeneous magnetization to the inhomogeneous Condon domain state (CDS) is defined as the point where a small irreversibility develops in the dHvA magnetization, as manifested by an extremely nonlinear response in the pickup voltage indicative of a threshold character. The third harmonic content in the ac response is used to determine with high precision the CDS phase boundary. The experimentally determined Condon domain phase diagram is in good agreement with the theoretical prediction calculated by the standard Lifshitz-Kosevich formula.

We demonstrate experimentally that the presence of a single domain wall in an underlying ferromagnetic BaFe12O19 substrate can induce a considerable asymmetry in the current (I)–voltage (V) characteristics of a superconducting Al bridge deposited on top of the substrate. The observed diodelike effect, i.e., polarity-dependent critical current, is associated with the formation of a vortex-free channel inside the superconducting area which increases the total current flowing through the superconducting bridge without dissipation. The vortex-free region appears only for a certain sign of the injected current and for a limited range of the external magnetic field.

A combination of scanning Hall microscopy and scanning ac-susceptibility measurements in superconducting stripes (ribbons) of width $wl10\text{ }\ensuremath{\mu}\text{m}$ was used to observe the dimensional phase transitions of the vortex lattice and its stability under alternating fields. At low dc magnetic fields applied perpendicularly to the plane of the stripes, vortices form a one-dimensional chain at the center of the stripes. Above a certain field ${H}^{\ensuremath{\ast}}(w)$, the vortex chain splits in two parallel rows displaced laterally in such a way that a zigzag vortex pattern is observed. By shaking the vortices with an external magnetic ac field and detecting their in-phase motion locally, we can identify the degree of mobility of each individual vortex. This technique allows us (i) to directly visualize the transition from intravalley (Campbell regime) to intervalley vortex motion as the amplitude of the ac modulation is increased and (ii) to accurately determine the temperature at which the vortex lattice freezes in a field-cooling experiment.

International audience; The Condon domain phase diagram for beryllium is determined in magnetic fields up to 10 T and at temperatures down to 1.3 K using a standard ac pick-up coil method to measure the de Haas-van Alphen (dHvA) effect. The detection of the transition point from the homogeneous state to the Condon domain state (CDS) is based on the extremely non-linear response to the modulation field resulting from a small irreversibility in the dHvA magnetization. The experimental results are compared with theoretical predictions calculated from the Fermi surface (FS) of beryllium. The width h_m of the hysteresis loop in the CDS is measured in a wide temperature and field region. A model for the hysteresis size is proposed and numerically calculated for the whole phase diagram.

We demonstrate experimentally and theoretically that the dissipative state of superconducting samples with a periodic array of holes at high current densities consists of flux rivers resulting from a short-range attractive interaction between vortices. This dynamically induced vortex-vortex attraction results from the migration of quasiparticles out of the vortex core (kinematic vortices). We have directly visualized the formation of vortex chains by scanning Hall probe microscopy after freezing the dynamic state by a field cooling procedure at a constant bias current. Similar experiments carried out in a sample without holes show no hint of flux river formation. We shed light on this nonequilibrium phenomena modeled by time-dependent Ginzburg-Landau simulations. ispartof: Physical review letters vol:104 issue:1 pages:1-4 ispartof: location:United States status: published

Using the highly inhomogeneous fields of a magnetic substrate, tunable junctions between superconducting and normal-state regions were created inside a thin-film superconductor. The investigation of these junctions, created in the same material, gave evidence for the occurrence of Andreev reflection, indicating the high transparency of interfaces between superconducting and normal-state regions. For the realization of this study, a ferromagnet with magnetic-stripe domains was used as a substrate, on top of which a superconducting transport bridge was prepared perpendicular to the underlying domains. The particular choice of materials allowed to restrict the nucleation of superconductivity to regions above either reverse domains or domain walls. Moreover, due to the specific design of the sample, transport currents in the superconductor passed through a sequence of normal and superconducting regions.

A thin superconducting (S) film reduces the mobility of magnetic domains in an underlying ferromagnet (F). The magnetic domain pattern of a barium hexaferrite single crystal was imaged as a function of temperature and external magnetic field, in both presence and absence of a covering niobium film. Comparative analysis of these two cases reveals that the relaxation of the domains is hampered, if the external field is changed while the Nb-film is superconducting. This effect in the S/F hybrids is interpreted in terms of a vortex-mediated pinning of magnetic domains by pinning centers in the superconductor.

A direct visualization of the flux distribution in a Pb film covering a five-fold Penrose array of Co dots is obtained by mapping the local field distribution with a scanning Hall probe microscope. We demonstrate that stable vortex configurations can be found for fields H~0.8H_1, H_1 and 1.6H_1, where H_1 corresponds to one flux quantum per pinning site. The vortex pattern at 0.8H_1 corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6H_1 the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H=1.6H_1 interstitial and pinned vortices arrange themselves in ring-like structures ("vortex corrals") which favor the formation of a giant vortex state at their center., Comment: 4 pages, 4 figures

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high resolution imaging techniques: Bitter decoration and scanning Hall probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field in agreement with the results of simulations. Real space vortex imaging has revealed that, in the onion state, the corners of the square rings act as effective pinning centers for vortices., Comment: To be published in Phys. Rev. B

A hysteresis loop is observed in the de Haas‐van Alphen (dHvA) effect of beryllium at low temperatures for a quantizing magnetic field applied parallel to the hexagonal axis of the single crystal. The irreversible behavior of the magnetization occurs at the paramagnetic part of the dHvA period in conditions of Condon domain formation arising due to the sufficiently strong dHvA amplitude. The resulting extreme nonlinear response to a very small modulation field offers the possibility to find, in a simple way, the Condon domain phase diagram.

Micro Hall probes were used to measure the induction distribution on a single crystal surface under strong Landau quantization in magnetic fields up to 10 T and temperatures up to 1.3 K. The Condon domain structure has been directly observed on silver. Information about size and shape of the domain structure was obtained by analyzing Hall probes placed along and across the long sample axis and by tilting the sample. For beryllium, almost no induction inhomogeneity was observed although the expected effect should be at least ten times bigger than in silver.

A hysteresis loop is observed for the first time in the de Haas-van Alphen (dHvA) effect of beryllium at low temperatures and quantizing magnetic field applied parallel to the hexagonal axis of the single crystal. The irreversible behavior of the magnetization occurs at the paramagnetic part of the dHvA period in conditions of Condon domain formation arising by strong enough dHvA amplitude. The resulting extremely nonlinear response to a very small modulation field offers the possibility to find in a simple way the Condon domain phase diagram. From a harmonic analysis, the shape and size of the hysteresis loop is constructed., Comment: 4 pages, 5 figures, submitted to PRL

Using a set of micro Hall probes for the detection of the local induction, the inhomogeneous Condon domain structure has been directly observed at the surface of a pure silver single crystal under strong Landau quantization in magnetic fields up to 10 T. The inhomogeneous induction occurs in the theoretically predicted part of the H-T Condon domain phase diagram. Information about size, shape and orientation of the domains is obtained by analyzing Hall probes placed along and across the long sample axis and by tilting the sample. On a beryllium surface the induction inhomogeneity is almost absent although the expected induction splitting here is at least ten times higher than in silver., 4 pages, 6 figures, submitted to PRL

We report the realization of Ge single-hole transistors based on Al-Ge-Al nanowire (NW) heterostructures. The formation of these axial structures is enabled by a thermally induced exchange reaction at 350 °C between the initial Ge NW and Al contact pads, leading to a monolithic and monocrystalline Al-Ge-Al NW. The 25 nm-diameter Ge segment is a quasi-1D hole channel. Its length is defined by two abrupt Al-Ge Schottky tunnel barriers. At low temperatures, the device shows a single hole transistor signature with well pronounced Coulomb oscillations. The barrier strength between the Ge segment and the Al leads can be tuned as a function of the gate voltage VG. It leads to a zero conductance at VG= 0 V to a few quantum conductance at VG= –15 V. When the gate voltage increases from –5 V to –3 V, the charging energy is extracted and it varies from 0.39 meV to 2.42 meV.We report the realization of Ge single-hole transistors based on Al-Ge-Al nanowire (NW) heterostructures. The formation of these axial structures is enabled by a thermally induced exchange reaction at 350 °C between the initial Ge NW and Al contact pads, leading to a monolithic and monocrystalline Al-Ge-Al NW. The 25 nm-diameter Ge segment is a quasi-1D hole channel. Its length is defined by two abrupt Al-Ge Schottky tunnel barriers. At low temperatures, the device shows a single hole transistor signature with well pronounced Coulomb oscillations. The barrier strength between the Ge segment and the Al leads can be tuned as a function of the gate voltage VG. It leads to a zero conductance at VG= 0 V to a few quantum conductance at VG= –15 V. When the gate voltage increases from –5 V to –3 V, the charging energy is extracted and it varies from 0.39 meV to 2.42 meV.